Antennas are used to radiate or receive radio wave signals. The transmission and reception of radio wave signals is useful in a broad range of activities. For instance, in the overland shipping industry, it is desirable to be able to track the exact location of goods or materials while in transit from a central location. In addition, it is desirable to be able to direct the performance of certain functions with respect to such goods from a central location. However, it is important that any antenna used in connection with such an application have a low profile so that it does not interfere with the normal functioning of, for example, the trailer or shipping container to which the antenna is affixed.
Increasingly, raw materials and finished goods are shipped by tractor trailer. A typical tractor trailer, or semi, consists of a tractor to provide motive force, and a trailer of approximately 40 feet in length that is supported at a front end by the tractor. Tractors and trailers are equipped with a standard hitching mechanism to allow interchangeability between a large number of tractors and trailers. In addition to being towed by tractors, trailers may be placed on specially adapted rail cars. Therefore, in a typical scenario, a trailer may be loaded with goods at a factory and hitched to a tractor for transport to a rail facility. The trailer may then be uncoupled from the tractor and placed on a rail car. The rail car may then carry the trailer to another rail depot where the trailer may be lifted from the rail car and interconnected with a second tractor for transport to its final destination
Because of the large carrying capacity of modern trailers, and because of the large number of goods being transported in such trailers, shipping companies and the owners of goods and materials transported by tractor trailers require precise and up to date information regarding the location of their goods and materials while they are in transit. Existing methods for tracking tractor trailers include noting the location of the trailers as reported by the drivers operating the tractor to which a trailer is attached. However, such information is only as timely as the last report received from the driver. Also, such a system relies entirely upon the driver to accurately report the position and condition of the goods or materials. Furthermore, such systems offer no way for a company to track the whereabouts of a shipment that has been hijacked or stolen from a temporary storage facility.
One proposed method of tracking the location of trailers or shipping containers combines a global positioning satellite (GPS) system with a radio transmitter. Such a system allows position information as determined by the GPS receiver to be communicated to the radio transmitter, which in turn transmits the position information to a central site. Also, the transmitter may be used to communicate information regarding such things as the ambient temperature inside the trailer. Systems combining GPS receivers and radio transmitters may additionally include a radio receiver. The provision of a radio receiver allows certain commands to be transmitted from the central control site to individual trailers. Therefore, commands may be sent from the central control site to, for example, operate the refrigeration unit on trailers equipped to carry refrigerated goods. With such a system, the shipping company need not rely on the driver to operate the refrigeration unit. Another example of the usefulness of a radio receiver on a trailer is to allow central control of the locking and unlocking of a trailer. Yet another example of the advantages offered by a transmitter and receiver on a trailer is the provision of revised routing instructions to the driver through a computer interconnected to the antenna.
Where a radio antenna is to be provided on a trailer, it is desirable that such an antenna have a low or very low profile. In a typical trailer having an enclosed space for containing the items to be transported and thereby protecting them from theft and the elements, the trailer is as tall as possible given the constraints imposed by overpasses, tunnels, and applicable laws. Accordingly, any antenna structure affixed to such a trailer must not add appreciably to the trailer's height. In addition, such trailers must be capable of reliable operation in all types of weather. Therefore, it is desirable that an antenna affixed to the exterior of a trailer be protected from the elements. A further desirable attribute of an antenna to be placed on the exterior of a trailer is that it not have a deleterious effect on the aerodynamic drag of the trader. All of these requirements are met by an antenna having a low or very low profile. Furthermore, these requirements are advanced by placing the antenna within an enclosure.
With the increasing concern for saving fuel, the aerodynamics of tractor trailers have received more and more attention. One commonly adapted measure to improve the aerodynamics of enclosed trailers is to provide a sloping top surface. Therefore, trailers are commonly provided with a top that rises approximately one inch from the leading edge of the trailer to approximately 14 feet behind the leading edge of the trailer. Accordingly, at the front edge of the trailer, there is an area that is approximately one inch below the highest extent of the trailer. Therefore, if an antenna unit having a height of about one inch or less were provided, it would not add appreciably to the height of the trailer. In addition, an antenna having a height of one inch or less would have little effect on the aerodynamics of the trailer. An antenna having a small height with respect to the operating wavelength is generally known as a very low profile antenna (VLPA).
Although placing the antenna within an enclosure or radome protects the antenna from the elements and helps maintain the aerodynamics of the trailer, these enclosures affect the tuning of the antenna. Also, the placement of an antenna on a large surface, such as on the top of an enclosed trailer, also affects the tuning of the antenna. Therefore, it would be advantageous to provide a low profile or very low profile antenna that could be tuned with its radome in place. Furthermore, it would be desirable to provide such an antenna that could be tuned with consideration given to the effect the antenna's ultimate operating environment has on its tuning.
Antennas having a low or very low profile generally have a very high Q value. An antenna with a high Q value has relatively high sensitivity over a relatively narrow range of frequencies. Therefore, for such an antenna to be adequately sensitive over its intended useful frequency range, it must be precisely tuned. Generally, the operating frequency of a patch antenna element can be altered by altering the length of the element or by altering the height of the element above the ground plane.
According to an existing method for tuning antennas, the length of the antenna elements are trimmed. However, this method of tuning antennas is tedious and time consuming. Also, this method produces a large amount of waste, resulting in a messy assembly area. Furthermore, this method of tuning antennas is irreversible; an antenna that has been over-trimmed cannot be made to meet the required scations, and must generally be discarded. Additionally, such designs are incapable of being tuned with an associated radome in place, and therefore require that the person tuning the antenna anticipate the effect that installation of a radome will have on the useful frequency range of the antenna.
Other existing antenna designs that provide a tuning mechanism to obtain optimal performance have required a large number of additional components to provide tunability. For example, some such designs provide an additional ground plane, the distance of which from the antenna element is adjustable using screws and wing nuts. However, the addition of such components results in an antenna having a greatly enlarged size and complexity. In other designs, dielectric material is placed in close proximity to the antenna element to vary the load on that element and thereby tune the antenna. Such designs introduce additional complexity and result in unnecessary electrical losses. These designs are also too large for use in very low profile antenna applications.
Another existing design incorporates ribbon antenna elements located adjacent to a ground plane. One end of the ribbon-shaped element is affixed to the ground plane, while the other is held above the ground plane by a nonconductive screw. The height of the element above the ground plane may be varied by adjustment of the screw. This design requires that the ribbon element be flexible, to prevent binding of the tuning screw during the tuning process. In addition, these designs do not provide a way to tune the antenna with a radome in place. Furthermore, this design provides no protection against short circuits when the tuning screw is adjusted so that the ribbon element is in very close proximity to the ground plane. When used in an environment with significant amounts of vibration, the ribbon element can move away from the head of the tuning screw, thus allowing the antenna to lose its tuning.
For the above-stated reasons, it would be advantageous to provide a low profile or very low profile antenna that is capable of being tuned with high precision. In addition, it would be advantageous to provide such an antenna that is highly resistant to de-tuning or failure due to vibrations in the antenna's operating environment. Concomitantly, such an antenna must be reliable, inexpensive to manufacture, and easily tuned.